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Eye On Electronics

A vehicle owner’s misguided attempt to use aluminum foil to squelch radio frequency interference (RFI) emanating from the ignition system provides an opportunity to learn how RFI shielding is supposed to work.

Sometimes you see things that make you just want to shake your head. The item in mind at the moment is a Ford coil-on-plug (C-O-P) coil I saw recently. The vehicle owner had wrapped all eight of the coils on his engine with a copper-colored aluminum foil. When I asked about this, the fellow told me he was trying to suppress the radio frequency interference (RFI) coming from the coil that was interfering with his radio’s reception.

What he did won’t work, at least for his purpose. It is, though, a good starting point for a discussion about how RFI suppression really is accomplished and why his idea doesn’t get there.

Among the early heroes of electrical discovery is a fellow by the name of Michael Faraday. He was a self-educated man who was an active scientist in England during the first half of the 1800s. The unit of capacitance, the farad, was named after him. Over his long career, he made many basic discoveries of magnetic science that we still use today.

One of Faraday’s discoveries was something we now call the Faraday shield. What he found out was that a metallic shield wrapped around a device would prevent static magnetic fields from affecting the device. The static magnetic fields rearrange the magnetic fields on the surface of the shield. This cancels the static fields and prevents them from affecting the protected circuit. Grounding the shield drains away the AC currents that collect on the shield, making it more effective as an electromagnetic shield. Faraday shields are used to protect ECMs and other sensitive electronics.

Our friend with the aluminum foil was trying to do the same thing, only in reverse. He was hoping to protect the outside world from the magnetic fields existing inside that ignition coil. The trouble is that the frequencies of the coil’s operation are not relevant from an RFI standpoint.

The main thing to tell our foil-on-the-coil friend is that the reason his idea doesn’t work is because the noise he’s trying to suppress doesn’t come from the coil. What we call radio frequency interference really comes from the spark plug gap when it’s firing. Here is how that works:

Think of the spark plug as a capacitor. The basic structure of a capacitor is two or more plates separated by insulators. The amount of capacitance depends on the number of plates, the area they occupy, the distance between the plates and the dielectric constant of the insulating material.

Now, compare those requirements with the construction of an actual spark plug. The single ground leg and the single center electrode of the plug form the two plates of the capacitor. Those tiny surfaces don’t occupy very much area. Normally, plug gaps are in the range of 1.5 to 2.0mm. Since capacitance goes down as the distance between the plates goes up, you can see we’re going to have a very small value of capacitance. In actual capacitors, the distance between the plates can be as “thick” as a film of plastic or as thin as a layer of oxide used to insulate the plates of an electrolytic capacitor. The spark gap is huge in comparison.

In this model of the spark plug as a capacitor, the insulating material “between the plates” is the fuel and air of the combustion chamber. It’s often mixed with some trace materials, such as water vapor and exhaust gases, that were not completely removed from the cylinder during the previous exhaust stroke. As it turns out, this explains why it takes more voltage to fire a plug with a rich mixture vs. a lean one, high compression vs. low compression and heavy loads vs. light loads. These factors all affect the “insulation” of the fuel and air that’s in the spark plug gap.

So what actually happens at the spark plug gap is that the voltage applied from the coil creates an electrostatic field in the insulating material that rapidly rises in intensity. When the forces applied to the molecules in the gap reach the required level, electrons are ripped loose from the outer shells of the atoms, creating atoms with a net positive charge. This is called ionization. Once an ionization path is created from one electrode to another, current will flow. This current, flowing through the resistance of the gap, creates the heat that starts the combustion process.

In the process of breaking the gap, something else happens that is of major importance. The sudden flow of current, combined with the tiny capacitances distributed throughout the system, creates high-frequency oscillations that want to radiate from the spark gap. These oscillations cover a broad portion of the entire radio frequency spectrum. It’s this noise radiating from the spark gap that produces the interference.

So what do the carmakers do about it? There are at least three basic strategies that revolve around the idea of being able to suppress the spark noise at its source. If the noise were allowed to radiate back toward the coil, it could create several different kinds of trouble. It could radiate from the vehicle wiring and cause trouble with other electronic circuits such as the ECM and other electronic assemblies.

The energy reflecting back from the spark plug also can create a standing wave problem, like a CB radio with a bad antenna. These standing waves can reach voltages that could reenter the coil, damaging its insulation.

The first line of defense is in the spark plug itself. In series with the plug is a resistor of about 5K ohms that’s mounted inside the spark plug. The plug itself has a capacitance relative to ground. The noise energy from the gap finds it easier to go off to ground through this capacitance than back toward the coil through the resistor. The take-away here is to never install nonresistor plugs into a vehicle designed to have resistor plugs in it. That 5K is important to the survivability of the coil and its electronics.

The second line of defense is the lead from the plug to the coil. Depending on the application, there are a couple of approaches to this. In the case of the C-O-P design, this link is a spring with a ferrite rod positioned in the middle. There are a specific number of the spring coil’s turns located over a certain length of the ferrite rod. This makes the link frequency-sensitive. The ferrite rod in this case is called a lossy material. This means that when the RFI energy enters the ferrite, it’s attenuated, or absorbed, by it. Stealth airplanes are covered in ferrite materials to help absorb incoming radar.

Conventional distributor ignition systems have resistor-type spark plug wires. They’re designed to be really poor radiators of the spark energy. The typical suppressor wire has a core material similar to the aramid fiber used in tires. It’s covered in a conductive graphite material. The graphite conducts the spark energy to the plug at a resistance of about 5K ohms per foot.

An additional factor is that the graphite core also serves as one plate of a capacitor while the engine block serves as the other. When the RFI energy comes back from the spark plug, the resistor cable wire is not only a poor antenna, but it’s a capacitor that shunts off the high-frequency noise to ground rather than allow it to travel against the impedance of the wire.

It’s important to remember we’re dealing with high-frequency AC noise. The impedance of the capacitor is infinite for DC, but very low, depending on how high the frequency is.

Something of an additional factor is that the engine parts surrounding the gap and the spark plug also serve to attenuate the radiating noise. All of the systems contribute to RFI suppression, but are not necessarily 100% effective.

One additional strategy is to use a bypass capacitor on the B+ line to the coil from the ignition switch. This is a solid copper line and would make a good radiating antenna if some of the RFI noise were to get past the above-mentioned defenses and cross from the secondary into the primary. This cap is usually a .22mfd cap rated at 400 volts. What happens here is that the noise from the spark gap is AC while the B+ line is intended to carry DC voltage only. The cap takes any AC component and sends it off to ground before it can spread by radiation.

So, what do you do when you suspect that a vehicle has RFI problems? The first step is to check your RFI defense measures and make sure they’re all in place and doing their jobs.

There are at least three or four good first places to look. In the case of a vehicle with spark plug wires, check the terminals at both ends. If these are loose, an additional spark from the wire to the terminal might be generated. The only spark is supposed to be at the plug. Be sure to check that the .22mfd capacitor has not come loose from the B+ side of the coil. Often this cap never gets hooked back up after disassembly. People don’t know why it’s there and often fail to reconnect it. Another great place to look is to make sure the right plugs are in the engine. By accident or by ignorance, the wrong plugs, especially if they’re the nonresistive type, can cause a lot of trouble.

Still another place to look is the spring inside the boot. That spring is meant to be latched to the output terminal of the coil on one end. The other is to be pressed over the end of the external spark plug terminal. Remember that this is held in place by compressing the length of the spring. It’s not necessary to bend or distort the diameter of the spring to hold it on at either end.

One more point is worth making relative to the aluminum foil-on-the-coil concept. Not only does it not work, it can actually make some things worse. The output of the coil is a function of the secondary capacitance the coil sees. Adding a foil layer actually could reduce the output of the coil. The coil also depends on the seal between the spark plug boot and the plastic of the housing to hold on to the 35- to 40Kv possible output of these coils. If the foil gets in between the boot and plastic housing, it could reduce the high-voltage insulation.

Lastly, the coil itself depends on layers of epoxy and plastic for its electrical insulation. Nothing that’s at ground potential—especially nothing sharp or pointed—should be mounted within 6mm of the coil. It’s designed not to have a shield on the outside. It’s also designed to have no sharp edges nearby that could attract voltages that might break the insulation. Not only does the foil not work, it represents the worst-case condition for the coil’s insulation. It’s likely to eventually wind up killing the coil.